1. Field of the Invention
The present invention relates generally to a tunable filter module which can vary the center wavelength in its passband, and more particularly to a tunable filter module which can automatically control the center wavelength in its passband to the center wavelength of input light according to variations in wavelength of the input light.
2. Description of the Related Art
Attention is now being given to a wavelength multiplexing transmission system for transmitting many optical signals having different wavelengths by the use of a single optical fiber as a system capable of obtaining a very large transmission capacity and a high cost performance. In general, the wavelength multiplexing transmission system employs a filter for passing signal light having object wavelengths to a receiving side and cutting off light having the other wavelengths.
In general, it is preferable that an optical filter has a narrow passband of wavelengths. However, the wavelengths of signal light vary with temperature changes or aged deterioration of a light source on the transmitting side of an optical communication system. Therefore, in the case that the passband of an optical filter to be used is too narrow, there is a possibility that signal light may also be cut off by the optical filter. To realize a system for controlling the center wavelength in the passband of the optical filter in response to a change in wavelength of the signal light, a tunable filter module capable of varying the center wavelength in its passband has now been desired.
Conventionally known is a tunable filter module which can vary the center wavelength in its passband by mechanically changing the angle of incidence on an optical interference film or a diffraction grating. In such a conventional tunable filter module, if the set wavelength of the tunable filter module (the center wavelength in the passband of the tunable filter module) deviates from the center wavelength of the signal light, it is necessary to correct the set wavelength of the tunable filter module to a shorter-wavelength side or a longer-wavelength side. However, the direction of correction of the set wavelength cannot be determined.
To cope with this, the set wavelength of the tunable filter module is conventionally varied with a given period, and the intensity of transmitted light is monitored to obtain the center wavelength in the passband. However, such a periodic change in the set wavelength of the filter module causes an adverse effect on signal light such as variations in intensity of the signal light, thus increasing an error rate. Further, other defects include time consumption in an operation for finding out the center wavelength in the passband and complexity of a control system.